Square's guides and libraries in particular were a great place to start when last I worked on Android (2017). They filled in a lot of dumb gaps in basic, common functionality, and their guides and blog posts on Android topics generally left me feeling more confident in them than in Google's Android team. They were among the ones calling "the Emperor has no clothes!" over fragments, for instance, and they described and built alternatives.
It does look like they've abandoned a couple of their big view-related Java libs, which makes me wonder what they're using now. Seems they've switched to Kotlin for a lot of stuff.
Don't remember their blog being on Medium (ick) but it is now, looks like:
https://medium.com/square-corner-blog